bis(2-Phenylpyridinato)-[4,4′-bis(iodoethynyl)-2,2′-bipyridine]-iridium(III) Hexafluorophosphate

Abstract

This work presents the synthesis and structural characterization of a novel type of biscyclometalated Ir(III) complex, which is equipped with two iodoethynyl moieties on its 2,2′-bipyridine (bpy) ligand. Iodoethynyl moieties represent prominent donor systems for the formation of supramolecular structures via halogen bonding (X-bonding). The synthesis of bis(2-phenylpyridinato)-[4,4′-bis(iodoethynyl)-2,2′-bipyridine]iridium(III) hexafluorophosphate, (2)(PF₆), is straightforward and involves post-complexation iodination, thus expanding the already rich toolbox for performing “chemistry on the complex”. The formation of the iodoethynyl moieties was unequivocally proven by ¹H-NMR spectroscopy, ESI-TOF mass spectrometry, and single-crystal XRD analysis.

1. Introduction

Halogen bonding (X-bonding) represents one type of non-covalent interaction, which is utilized in today’s supramolecular chemistry [1]. Although the principle, i.e., the non-covalent interaction between halogens and Lewis bases, was already known in the 19th century [2,3], the enormous potential of X-bonding was overlooked for a long period of time. Still, X-bonding represents the least-explored non-covalent force, in the context of solution chemistry [1]. In recent years, significant progress has been made to realize the step from fundamental research to applications [4]. For example, the self-healing of cross-linked polymers, due to the reversibility of X-bonding, was reported by Hager, Schubert et al. [5,6]. Indeed, X-bonding represents a promising tool in the development of tailor-made polymers for various applications [7]. Moreover, X-bonding can also be used for crystal engineering, in which metal centers/complexes are assembled by the interaction of appropriate X-bonding donors and acceptors (Scheme 1a) [8]. The same principle also applies to other applications: liquid crystals, anion recognition, templated self-assembly, organo(metallic) catalysis, etc. [9,10].

From the wide range of donor–acceptor combinations, which might be utilized in either of these fields, the interaction between iodine atoms (due to the excellent polarizability) and Lewis bases has extensively been studied [11]. It has been elaborated that the electrophilicity of the iodine atom and thus its donor capability can be increased by linking it to electron-withdrawing moieties or sp-hybridized carbon atoms [11]. The iodoethynyl moiety has been attached mainly to benzene derivatives to yield supramolecular structures in combination with aliphatic or aromatic amines. However, metal complexes equipped with this X-bond-donor have—to the best of our knowledge—not yet been reported. Such complexes might be of importance regarding potential applications in anion sensing due to luminescence quenching/enhancement [12], (photo)catalysis [13,14], etc. Here, we present the synthesis and characterization of an archetypal complex, which combines emission properties, due to the biscyclometalated Ir(III) center, with presumptive X-bonding capability, due to its iodoethynyl arms.

2. Results

2.1. Synthesis and Characterization

The synthesis of the title compound was performed in a stepwise fashion starting from the well-known [Ir(ppy)₂Cl]₂ precursor complex and 4,4′-diethynyl-2,2′-bipyridine (Scheme 2). The reaction of these compounds in a 1:2 ratio afforded the Ir(III) complex (1)(PF₆) in 83% yield. It has to be noted that the synthesis of complex (1)(PF₆) had been reported by Grachova et al. beforehand [15]. However, in deviation from this procedure, the complexation was carried out with the desilylated bpy ligand without disturbance of the “free” ethynyl moieties. The subsequent iodination of 1 using AgNO₃ and NIS, as the electrophile, gave the bis(iodoalkyne) (2)(PF₆) in 59% yield [16].

Complexes 1 and 2 were thoroughly characterized by ¹H- and ¹³C-NMR spectroscopy as well as high-resolution electrospray ionization time-of-flight (ESI-TOF) mass spectrometry. The ¹H-NMR spectra of 1⁺ and 2⁺ featured the characteristic signal patterns known for complexes from the [(ppy)₂Ir(bpy)]⁺ family [17]. The successful formation of complex (2)(PF₆) was revealed by the disappearance of the signal of the ethynyl protons at ca. 4 ppm (Figures S1 and S2). Moreover, iodination altered the electronic properties of the bpy ligand. Thus, the signals of the bpy ligand experienced distinct shifts to lower frequencies. This upfield shift was most pronounced for the signals of the bpy protons in the 3,3′ and 5,5′ positions (Figure 1a). On the other hand, the signals, which were assigned to the protons of the cyclometaling ligands, were hardly affected. Similar effects were also observed when comparing the ¹³C-NMR spectra of compounds 1 and 2. In particular, the signal of the terminal alkyne C-atom was shifted from 79.73 to 24.32 ppm due to iodination (Figures S3 and S4). This pronounced upfield shift is characteristic of aromatic iodoethynyl derivatives [18].

For both complexes, high-resolution ESI-TOF mass spectra were recorded (Figures S5 and S6). Excellent agreement of the obtained m/z values with the calculated ones corroborated the successful synthesis of both complexes (Figure 1b). Besides the prominent peaks of the complex cations, i.e., 1⁺ and 2⁺, some minor peaks, which resulted from fragmentation reactions, were observed. In the case of 2⁺, one peak appeared at the m/z value of 831.06; this peak was due to the loss of an iodine atom. Moreover, the conversion of one ethynyl moiety to a CH₃ group was assumed. This behavior has been reported, as a side reaction, when dealing with Ru(II) complexes [19] and might also occur for 1⁺ and 2⁺ under the experimental conditions of ESI-TOF MS (albeit via a different mechanistic pathway). On the other hand, the often-observed decoordination of the bpy ligand was absent in both cases, at least in the range of the applied ionization energy.

2.2. Single-Crystal X-Ray Diffraction (XRD) Analysis

Single crystals suited for XRD analysis were obtained by slow diffusion of diethyl ether into a saturated solution of (2)(PF₆) in CH₃CN. The complex cation 2⁺ features the expected, almost perfect octahedral geometry, which is known from the parent [(ppy)₂Ir(bpy)]⁺ cation and many of its derivatives (Figure 2a) [20,21]. The bond lengths are similar to those reported for [(ppy)₂Ir(bpy)]⁺ [17]. The elongation of the Ir–N_ppy and Ir–C_ppy bonds is ascribed to the presence of the iodoethynyl moieties on the bpy ligand (

Table 1. Selected bonding parameters of the complex cation 2⁺.

Bond	Length (Å)	Length (Å)	Bond	Angle (°)	Angle (°)
Ir–Nbpy	2.135(5)	2.132(3) 1	Nppy–Ir–Nppy	171.4(3)	172.1(1) 1
Ir–Nppy	2.057(5)	2.044(3) 1	Nbpy–Ir–Cppy	171.1(2)	171.8(1) 1
Ir–Cppy	2.019(6)	2.014(4) 1			147.9(2) 1
C≡C	1.209(10)	1.179(8) 2	Nbpy–Ir–Nbpy	76.5(3)	76.2(1) 1
I–C	1.985(7)		Nppy–Ir–Cppy	80.2(2)	80.7(1) 1
					80.1(1) 1
			I–C≡C	177.8(7)
			C≡C–Cbpy	179.0(8)

¹ Bond lengths and angles in the parent complex [(ppy)₂Ir(bpy)](PF₆) [17]. ² Bond length in a Re(I) complex, which contained a 4,4′-diethynl-bpy ligand [22].

). The trans-donor angles are all ca. 171°; on the other hand, the corresponding angles of [(ppy)₂Ir(bpy)]⁺ cover a broader range (i.e., 172 to 175°). Moreover, the bite angles of the bpy ligand and the ppy^– ligands are similar to those in the parent complex (i.e., ca. 76° and 80°, respectively). Remarkably, the C≡C triple bond is significantly longer than that in a related Re(I) complex, which contained the 4,4′-diethynyl-equipped bpy ligand, or a Ag(I) complex, which carried 4-(iodoethynyl)phenyl moieties on its dipyrrine chelates [22,23]. Thus, the bond elongation is unequivocally not solely due to the iodination. The considerable deviation from linearity within the iodoethynyl moieties has also been observed in other complexes, which contain such lateral functionalities [23]. The rings of the bpy ligand are not co-planar but show a slight torsion of ca. 2.5°. Selected bonding parameters are summarized in Table 1, and the full data are presented in Tables S1 and S2.

In the crystal lattice, the 2⁺ cations are organized in orthogonal zig-zag chains with the iodoethynyl moieties of each two chains pointing towards each other (Figure 2b). However, these moieties are not involved in H-bonding or X-bonding interactions. Furthermore, the arrays of 2⁺ cations are separated by sheets of PF₆^– anions, which are oriented in the crystallographic a and b directions (Figure S7). In this respect, the solid-state structure of (2)(PF₆) resembles that of [(ppy)₂Ir(bpy)](PF₆) [17].

3. Discussion

Regarding the synthesis of the title compound, two notable aspects have to be addressed: Firstly, the precursor complex (1)(PF₆) was prepared by the coordination of 4,4′-diethynyl-bpy to the biscyclometalated Ir(III) center. Thus, the complexation was not affected by the ligand’s terminal alkyne moieties, which are known to be hardly compatible with Ru(II) centers, as present in common precursors from the (bpy)₂RuCl₂ or (tpy)RuCl₃ families. In those cases, side reactions reduce the yield, and the use of silyl-protected triple bounds is highly advisable [19,24], and ethynyl-equipped Ru(II) complexes have only been obtained in very few cases [25]. Secondly, the “chemistry on the complex” concept was applied to prepare (2)(PF₆) by electrophilic iodination in the presence of a Ag(I) salt. To the best of our knowledge, this represents the first example of the iodination of an ethynyl-equipped transition-metal complex.

However, preliminary attempts to use the title compound as an X-bond donor were not successful (i.e., crystallization of (2)(PF₆) in the presence of pyrimidine bases). Further studies to utilize (2)(PF₆) for the assembly of supramolecular structures via X-bonding are currently ongoing.

4. Materials and Methods

4.1. General

All chemicals were purchased from BLDpharm (Reinbek, Germany), Sigma-Aldrich (Taufkirchen, Germany), and TCI (Eschborn, Germany) and were used as received unless stated otherwise. [Ir(ppy)₂Cl]₂ and 4,4′-diethynyl-2,2-bipyridine were prepared as described elsewhere [20,26]. NMR spectra were recorded on a Bruker 400 MHz Advance III spectrometer (Karlsruhe, Germany) equipped with a cryo-probehead. Samples were measured in deuterated solvents from Euriso-Top (Saarbrücken, Germany). For the ¹H NMR spectra, a line broadening of 0.3 was applied. Chemical shifts are reported in ppm, referenced to the residual solvent signal. For the processing and analysis of the NMR spectra, the web-based NMRium software was used [27,28]. High-resolution electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) was conducted using a Bruker Daltonics MICROTOF II mass spectrometer.

4.2. bis(2-Phenylpyridinato-)(4,4′-diethynyl-2,2′-bipyridine)iridium(III) Hexafluorophosphate(1)(PF₆)

A solution of [(ppy)₂IrCl]₂ (37.85 mg, 0.0353 mmol) and 4,4′diethynyl-2,2′-bipyridine (14.42 mg, 0.0706 mmol) in a N₂-purged MeOH/CH₂Cl₂ mixture (1:2 ratio, 10 mL) was heated to 50 °C for 2 h whilst stirring. After cooling to room temperature, solid NH₄PF₆ (tip of a spatula) was added, and stirring was continued for 10 min. The solvents were removed in vacuo. The solid residue was dissolved in CH₂Cl₂ (2 mL), and insoluble particles were removed by filtration. Overlaying the solution with diethyl ether resulted in the precipitation of the crude product after standing at 5 °C for 12 h. Purification by column chromatography (Al₂O₃,CH₂Cl₂) gave the analytically pure complex as a red powder (49.82 mg, 83%).

¹H-NMR (400 MHz, CD₃CN) δ_H 8.66 (s, 2H), 8.07 (d, ³J_HH 8.26 Hz, 2H), 7.97 (d, ³J_HH 5.65 Hz 2H), 7.89 (m, 2H), 7.83 (d, ³J_HH 7.72 Hz, 2H), 7.62 (d, ³J_HH 5.83 Hz, 2H), 7.55 (d, ³J_HH 5.65 Hz, 2H), 7.12–7.01 (m, 4H), 6.95 (t, ³J_HH 7.41 Hz, 2H), 6.27 (dd, ³J_HH 7.54 Hz, ³J_HH 1.26 Hz, 2H), 4.05 (s, 2H) ppm. ¹³C-NMR (75 MHz, CD³CN) δ_C 167.82, 156.19, 151.27, 150.23, 149.49, 144.57, 139.26, 133.65, 132.01, 131.35, 131.00, 128.00, 125.48, 124.14, 123.32, 120.51, 87.61, 79.73 ppm: HRMS (ESI+) m/z (%) Calcd. for C₃₆H₂₄IrN₄ 705.1627; found 705.1615.

4.3. bis(2-Phenylpyridinato-)[4,4′-bis(iodoethynyl-2,2′-bipyridine]iridium(III) Hexafluorophosphate(2)(PF₆)

A solution of NIS (21.6 mg, 0.096 mmol) in acetone (3 mL) was added to a solution of (1)(PF₆) (34 mg, 0.040 mmol) and AgNO₃ (13.5 mg, 0.080 mol) in acetone (3 mL). The solvent volume was reduced to 4 mL whilst purging the solution with N₂. Subsequently, the reaction mixture was stirred at room temperature for 15 h. The solvent was removed in vacuo, and the residue was taken up with a H₂O/CH₂Cl₂ mixture (each 5 mL). The organic phase was washed with water (3 × 5 mL) and then dried over Na₂SO₄. The solvent was evaporated to afford the crude product. Finally, orange-colored single crystals suited for XRD analysis were obtained by the diffusion of diethyl ether into a saturated solution of the crude product (25.98 mg, 53%).

¹H-NMR (400 MHz, CD₃CN) δ_H 8,52 (s, 2H), 8.08 (d, ³J_HH 8.08 Hz, 2H), 7.93 (d, ³J_HH 5.74 Hz, 2H), 7.88 (m, 2H), 7.82 (d, ³J_HH 7.72 Hz, 2H), 7.60 (d, ³J_HH 5.65 Hz, 2H), 7.46 (d, ³J_HH 5.65 Hz, 2H), 7.11–7.02 (m, 4H), 6.94 (t, ³J_HH 7.30 Hz, 2H), 6.26 (d, ³J_HH 7.63 Hz, 2H) ppm. ¹³C-NMR (75 MHz, CD₃CN) δ_C 167.26, 155.57, 150.59, 149.76, 149.34, 143.99, 139.68, 133.95, 131.44, 130.87, 130.43, 127.53, 124.91, 123.56, 122.73, 119.93, 89.73, 24.32 ppm. HRMS (ESI+) m/z (%) Calcd. for C₃₆H₂₂I₂IrN₄ 956,9559; found 956.9541.

4.4. X-Ray Diffraction Analysis of (2)(PF₆)

The single-crystal X-ray intensity data for the reported compound was collected on a Bruker-Nonius Kappa-CCD diffractometer equipped with a Mo-Kα IµS microfocus source and an Apex2 CCD detector (Karlsruhe, Germany). Data were corrected for Lorentz and polarization effects; absorption was considered on a semi-empirical basis using multiple scans [29,30,31]. The structures were solved by direct methods (SHELXT [32]) and refined by full-matrix least squares techniques against Fo² (SHELXL-2018/3 [33]). All hydrogen atoms were included at calculated positions with fixed thermal parameters. All non-disordered, non-hydrogen atoms were refined anisotropically [33]. The crystal of (2)(PF₆) contains large voids, filled with disordered solvent molecules. The size of the voids was 129 ų/unit cell. Their contribution to the structure factors was secured by back-Fourier transformation using the SQUEEZE routine of the program PLATON [34], resulting in 65 electrons/unit cell. The structure representations were generated with the MERCURY software [35].

Crystal data for (2)(PF₆): [C₃₆H₂₂I₂IrN₄] 2[PF₆] [*], Mr = 1101.54 gmol⁻¹[*], light-yellow prism, size 0.062 × 0.054 × 0.048 mm³, monoclinic, space group C 2/c (no. 15), a = 20.4527(6), b = 14.3620(4), c = 15.7005(4) Å, β = 127.354(1)°, V = 3666.00(18) ų, T = −140 °C, Z = 4, ρ_calcd. = 2.003 gcm⁻³[*], µ (Mo-K_α) = 54.34 cm⁻¹[*], multi-scan, trans_min: 0.6235, trans_max: 0.7456, F(000) = 2080, 13843 reflections in h(−26/26), k(−18/18), l(−20/20), measured in the range 1.892° ≤ Θ ≤ 27.431°, completeness Θmax = 99.8%, 4191 independent reflections, R_int = 0.0457, 3690 reflections with F_o > 4σ(F_o), 247 parameters, 24 restraints, R1_obs = 0.0425, wR²_obs = 0.1037, R1_all = 0.0502, wR²_all = 0.1081, GOOF = 1.070, largest difference peak and hole: 1.402/−2.045 e Å⁻³. [*] derived parameters do not contain the contribution of the disordered solvent.